I really don't see how you get a good feel for mechanics in a party based game by playing solo, at least if the game is any good.
In a thousand ways. You learn basic stuff like how to move, how to attack, which is usually slightly different in every RPG. Example, in BT4 you can't hit a goblin with an axe chop unless your guy is in the same grid column. So from that you start to think about what types of "chess pieces" you want in your party: what mix of pawns, knights, and bishops? You get a feel for what % of hit points you're going to lose when a goblin hits you, which will inform how many "tanks" you need in the party. Do they one-hit kill? Do combats drag out over a dozen hits per monster? This will get you thinking about whether you want "alpha strike" characters, heal over time effects, low damage area attacks. You learn what cooldowns are and get a feel for which types of abilities might have longer or shorter ones. You can decide whether you want high power magic abilities with long cooldowns or fighters and rogues who can spam every turn. You learn whether or not fleeing from battle is a thing (in BT4 it's not) which might make you think twice about making lots of "glass cannon" characters. When you get your first magic-user you learn that spell points need to be charged up and they can't attack on the first round. So how many magic-users do you really want? You gain an understanding of how consumables work, or don't work. You learn there's no such thing as spamming healing potions which will inform your goals for support characters. Et cetera for days.
I don't even know if I like the "growing blob" model yet but its benefits are obvious.